This invention relates to color picture tubes of the type having a mask attached to a frame which is suspended in relation to a cathodoluminescent screen, and particularly to a support for suspending the mask-frame assembly within the tube.
In these color picture tubes, the accuracy with which the electron beams strike the individual elemental cathodoluminescent screen areas depends, to a great extent, upon the accuracy with which the mask apertures are aligned with the elemental screen areas during operation of the tube. Thus, as the mask expands by reason of thermal effects occasioned by the impact thereon of the electron beams, the resulting misalignment of the mask apertures and elemental screen areas causes a portion of the electron beams to misregister, that is, to impinge upon elemental screen areas other than the ones upon which they were intended to impinge.
Most present day color picture tubes utilize a bimetallic mask mounting assembly, such as described in U.S. Pat. No. 3,803,436, issued to Morrell on Apr. 9, 1974, to move the mask toward the screen, as the mask is heated, to compensate for mask expansion. In this patent, a bimetallic element is connected between a stud embedded in the faceplate panel and the mask electrode. The bimetallic element may be a spring welded directly to the frame or an intermediate member located between the spring and frame.
In a four spring support arrangement, wherein each spring has the same orientation, e.g., all extending either clockwise or counterclockwise relative to the mask-frame assembly, thermal expansion of the springs and frame causes the mask to rotate about the longitudinal axis of the tube. This rotation also causes misregister of the electron beams with the elemental screen areas.
In a three spring support arrangement, wherein the side springs both either extend upwardly or downwardly and a top spring extends toward either side, thermal expansion of the mask and springs cause shifts along the major and minor axes of the tube. Such shifts also cause misregister of the electron beams with the elemental screen areas.
To date, the aforementioned problems of rotation about the longitudinal axis and shifts along the major and minor axes have not been completely solved. The present invention provides improvement in the various spring support systems that at least minimizes rotation and shifts of the mask relative to the screen.